The present application relates to thermal management of an internal combustion engine, and more particularly, but not exclusively to evaporative cooling and wet compression of an internal combustion engine.
Engine knocking (also called detonation or spark knock and pinging) occurs in internal combustion engines when a portion of the air/fuel mixture in the cylinder does not combust with the rest of the air/fuel mixture in the cylinder at the precise time in the piston's stroke cycle as determined by the engine control system. The peripheral explosions create shock waves that can be heard by the operator. The effects of the engine knock shock waves can be of no consequence or catastrophic to the engine. In between these extremes, engine knock limits the compression ratio, ignition timing, and other engine operating parameters that effect efficiency on reciprocating internal combustion engines. Without engine knock, these parameters could be changed to enable more efficient operation and generally re-optimize the engine for superior performance.
Liquids can be used to cool combustion gases in internal combustion engines. The term wet compression refers to the act of vaporizing liquid during compression. The phase change from liquid to vapor consumes relatively large amounts of energy with a relatively small temperature change. Wet compression therefore allows a thermodynamic cycle that efficiently compresses an air/liquid mixture with a lower temperature increase than compressing dry air, enabling higher compression ratio and peak pressure at a constant peak temperature.
Humid air and water injection are related processes that provide some benefits to reciprocating internal combustion engines. Humid air cycles have been used in reciprocating engines and gas turbines to reduce NOx emissions. Water injection was used in WWII to increase aircraft engine power density, primarily for takeoff, but there are differences that allow wet compression to improve power density and efficiency relative to humid air and water injection.
Wet compression may be confused with humid air thermodynamic cycles. Humid air cycles inject steam or other non-reactive vapor into the air/fuel stream. Vapor injection increases the thermal mass of the air/fuel mixture and dilutes the charge air. The dilution decreases the tendency to knock while maintaining stoichiometric combustion. The larger thermal mass reduces the peak temperatures in the combustion chamber therefore reducing NOx formation.
Because humid air is already in vapor phase when mixed with the air/fuel stream, it cannot go through a phase change during compression. When humid air is compressed, the temperature increases almost as fast as if the air were dry. The temperature rise is slightly lower since the specific heat ratio of the vapor, e.g. water, is often lower than the specific heat ratio of the air/fuel mixture. Any improved knock margin is primarily dependant upon the dilution effect.
In water injection, large liquid droplets are added to an air/fuel stream to provide cooling from the phase change during compression and combustion. Upon injection, water droplets may follow one of two paths: collision with internal surfaces or entrainment in the airflow.
Large water droplets may collide with internal surfaces during airflow direction changes. The inertia of large water droplets can overwhelm the friction from surrounding airflow and hinder the droplet's ability to follow airflow changes around structures within the air ducting, intake manifold, and cylinder, causing droplet collisions with internal surfaces. If the surface is hot, such as in the cylinder, the liquid may vaporize, thus making the system act similar to humid air injection. If the droplet does not vaporize, it is pushed by the airflow along the internal surface. As it is moving, the liquid may coalesce with other liquid droplets. The ratio of surface area to volume decreases, which reduces the effects of air temperature on liquid vaporization. Surface wetting may also lead to engine corrosion issues such as cylinder liner scuffing and oil quality degradation.
If the droplets do not collide with a solid surface, they may remain in the airflow. Eventually, at least some of the droplets may arrive in the cylinder. During compression, the air temperature surrounding the droplets increases. Increased temperature causes heat to flow from the air to the water droplet. As the thermal gradient between the droplet and surrounding air increases, the heat transfer rate increases. Heat transfer across the large thermal gradients associated with large droplets increases system entropy, however, thus reducing cycle efficiency.
Sufficiently large droplets have enough thermal inertia to remain in liquid form during combustion. Large droplets have a lower surface area to volume ratio than small droplets. Because heat transfer depends upon surface area and heat required for total vaporization depends upon volume, large droplets may not have enough time to completely vaporize. When the large droplets are present during combustion, they lower peak cylinder pressures and temperatures and slow flame propagation which decreases system efficiency.
While water injection and humid air injection may have some ability to reduce engine knock, various shortcomings persist: entropy produced from large thermal gradients required during liquid vaporization; corrosion from surface wetting; and cooling the combustion process. Humid air cycles have temperature increases during compression equivalent to dry air compression limiting the efficiency of the combustion system. Thus, there is an ongoing demand for further contributions in this technical arena.